There is extensive literature describing the use of monomeric epoxy functional silanes. Such silanes are used either alone or combined with appropriate polymers. However, one of the main difficulties in the use of monomeric epoxy silanes in water is their sensitivity to hydrolysis and condensation which is difficult to control. In addition, the stability of the epoxy functionalities when using the monomeric epoxy silanes in water is difficult to control because of the tendency of the epoxy functionalities to exhibit ring opening.
The use of pre-hydrolyzed and pre-condensed silanes is one answer to such concerns. A pre-hydrolyzed and condensed silane can be an oligomeric structure that has specific features like controlled molecular weight, usually good film formation capabilities and dispersion properties because the silane terminations are already partially or totally condensed, and faster curing rates. This aspect of the oligomers makes them attractive to the coatings industry as it broadens the field of applications and also helps to get faster application or formulation properties. However, the high molecular weight oligomers can condense further to larger siloxane networks, which result in the formation of structures that are difficult to make water-soluble.
For example, U.S. Pat. No. 6,391,999 discloses multi-functional epoxy siloxane oligomers for use in a solventless or solvent-based system. These multifunctional epoxy siloxane oligomers have high molecular weights and an insignificant amount of residual silane functional groups. Thus, it is very difficult to make the oligomers water-soluble.
Another disadvantage of the use of monomeric epoxy silanes is that they release a large amount of volatile organic compounds (VOCs) expressed as alcohol content introduced by the alkoxy functionalities.
A general trend of the industry is to decrease or eliminate the release of VOCs or hazardous air pollutants (HAPS). It is desirable to reduce the methanol content of any structure that could be involved in coatings, adhesives and sealant applications.
It is also desirable to prepare water-based coatings, which are resistant to chemicals as well as corrosion resistant based on metallic powders like aluminum, zinc, bronze and other metallic or organic pigments. Metallic pigments being sensitive to water, there is also a need to have superior protection of such metallic powders in water against a well-known mechanism called hydrogen evolution.
It is also desirable to design water-based coatings that have superior adhesion properties, mechanical or chemical resistances with outstanding weathering behaviors and that can be applied on a variety of substrates such as metallic or plastic substrates, cellulosic or natural substrates, concrete and any other material generally used in the coatings and adhesives & sealant industries.
Therefore, there is a need to produce a water-soluble epoxy silane oligomer that is useful in a waterborne system. There is also a need for an epoxy silane oligomer structure having epoxy functional groups to be used in waterborne systems for corrosion protection, zinc rich primers, shop primers, metallic pigment dispersions or other coating applications.